1. Field of the Invention
The present invention relates to marine surveying equipment and towed seismic sensors known as streamers. More specifically, this invention relates to devices utilized to continuously traverse submerged streamers for the purposes of preventing adhesion of marine growth that can disrupt signal transfer fidelity of imbedded sensors and increase cable drag.
2. Description of the Prior Art
Marine seismic surveying utilizes a method of reflection seismology to create a model of the seabed surface and the physical properties underlying the seafloor. Typically, seismic sensor lines called streamers are towed behind a maritime vessel near the ocean surface. An external energy source produces an energy wave directed at the seafloor, which reflects towards the surface and is measured using imbedded sensors in the streamer cable. These sensors are typically hydrophone devices that can measure the reflected sound energy. After the data is gathered, a detailed map of the seabed and its constitutive structure can be modeled and analyzed. Typical applications for this type of surveying include creation of detailed maps of the seafloor and for mineral and deposit exploration, namely for locating possible offshore drilling sites for oil and natural gas extraction.
Streamers are typically very long, sometimes kilometers in length. Several streamers are towed behind a maritime vessel simultaneously for data acquisition. The streamers are generally comprised of an insulation fluid or gel with a specific gravity less than one to provide buoyancy. External streamer positioning devices are installed every few hundred meters along each streamer length in order to correct for cable positioning errors.
As the streamers are towed through the ocean water, is it common for marine biological life and nonliving organic material to cling to the streamer outer surface, especially in warmer water regions. The buildup of this material is commonly known as fouling, and comprises different macro and micro organisms, as well as inorganic deposits. The buildup of marine growth, such as barnacles and other organisms, can cause streamer fidelity loss and increased drag on the cable.
Traditional methods of removing fouling along streamer cables include different manual operations that impede continual surveying operations. These methods include retrieval of the cable onboard the vessel in order to remove fouling buildup or sending the streamers away to a facility for professional cleaning and servicing. These methods are both expensive and time consuming. They stop production data acquisition and result in surveying down time, adding significant opportunity cost to the seismic contractor. Labor costs for cleaning the streamers are also higher, along with associated risks with these manual operations. These include the opportunity for crew injury as the streamer is recovered from the water and manually cleaned, as well as the potential for damaging the streamer while being handled and cleaned.
Several devices have been developed to replace these traditional streamer cleaning methods. U.S. Pat. Nos. 7,145,833 and 7,409,919 to Hoogeveen describe streamer cleaning devices that utilize fluid motion along a streamer to propel a cylindrical cleaning device longitudinally and rotationally around the streamer. Exterior vanes act as propellers to drive the motion of the device in a single direction along the streamer, while an internal bristle device removes fouling buildup. While this device is useful for removing fouling, it is limited in its use. The device can only travel in one direction along the streamer, and cannot switch directions. The device travels only once down the length of the streamer before requiring retrieval after each pass, and cannot pass over newer style leveling devices. The mechanical complexity and size of these units is also important to note, as it can affect its usability. These devices are more costly and generate higher acoustic noise than smaller-sized devices for the same purpose. The interior bristles scrap the outer surface of the streamer cables to remove fouling, which generates considerable broad spectrum noise that is difficult to filter and interferes with streamer sensor operation during use.
U.S. Pat. No. 7,754,018, also to Hoogeveen, describes another streamer external cleaning device that utilizes fluid motion to provide motive power to the device as it moves along the length of the streamer. This device provides a direction switching capability, in which the pitch of external turbine blades is adjusted to change the translational direction of motion. This device, along with the two previously reported devices, is significantly complex and produces a large acoustic signal that can disrupt the fidelity of the streamer cable sensors. They also provide a tool that removes fouling after it has adhered to the surface of the streamer, which requires are a rigorous cleaning mechanism that generates considerable noise during operation.
While these devices and methods may be utilized to remove fouling from a streamer cable, an improved antifouling device is required that can eliminate the known drawbacks in the prior art.